The region in the Dreamlands where the Forsaken Caravanserai is located.
According to the Yellow King it is a dangerous area, not necessarily because of its vastness or its distance from Dreamlands civilization, but because the entire region is what he called the Transition, or the Thinning: it is where the Dreamlands ends. Whether the Dreamlands dissolves/transitions into the Dimension of Dreams proper, or simply just ceases existing, is unknown. But the Yellow King stated outright that if he and the party were to travel east from the Caravanserai (in other words, towards the Mad Poet‘s Oasis), the further they went, the likelihood would grow that he (the Yellow King) might cease to exist and they (the party) might lose their minds, or never wake up.
The Yellow King named the following places as potentially reachable by traveling the Eastern Desert from the Caravanserai: heading west could conceivably result in reaching Tsol, Gak, and Sarnath; heading east could lead to the Mad Poet’s Oasis, Irem, the Pool of Night, and the Night Ocean.
In Chapter 150 (Part 1), YK shared that it didn’t matter how well one prepared for a journey across the desert, because the Mad Poet’s Oasis was “not located at a point on a map, but rather at the darkest part of their souls,” past the point of no return. The only way to reach it was to arrive at the end of their rope.
Later that chapter, Barnabus, Dabwick, Dora, Grip, Vik, and the Yellow King crossed the desert, and it proved to be a journey of memory and hallucination, filled with sights of suffering and loss, until they were stripped down to their core essence. And at a certain point, the party discovered that they had truly left the Dreamlands and stepped “outside of Time” as they witnessed other travelers making the crossing from other moments in time.
First Reference: Chapter 102
Other Notable References: Chapter 150 (Part 1), Chapter 175